1. Field of the Invention
The present invention relates to a load driver, a charging control method of a power storage unit in the load driver, and a computer-readable recording medium with a program recorded thereon for allowing a computer to execute charging control. In particular, the present invention relates to a load driver with a power storage unit serving as a main power supply of a hybrid or electric vehicle, the load driver having a mechanism of charging the power storage unit when its power stored by charging is scarce at the startup of the system, and further relates to a charging control method of the power storage unit in the load driver and a computer-readable recording medium with a program recorded thereon for allowing a computer to execute charging control.
2. Description of the Background Art
Against the background of the currently burning issues concerning energy conservation and environment, the hybrid vehicle (hereinafter HV) and the electric vehicle (hereinafter EV) are receiving growing attention. These vehicles have such a power storage unit as power capacitor and secondary battery mounted thereon that is used as a power source for driving a motor and accordingly moving the vehicle.
When the power storage unit (hereinafter referred to as main power supply) serving as a power source of the vehicle is discharged to a considerable degree and accordingly the remaining capacity of the main power supply decreases, it is required to charge the main power supply by any appropriate means. In particular, if a power capacitor with a large amount of self-discharge is employed as the main power supply, it is likely, after a long nonuse period, that the system cannot be started due to shortage of the output voltage of the main power supply.
A long nonuse period of the vehicle, for example, decreases the power stored, by charging, in the main power supply, and thus makes it impossible to start the system. One conventional method employed in such a case for charging the main power supply converts an AC voltage from an external commercial AC power supply into a DC voltage by an external charger in order to charge the main power supply.
The above-mentioned vehicles include, in addition to the main power supply, an auxiliary battery for supplying electric power to auxiliary loads such as lamp, small-sized motor and control circuit. The auxiliary battery corresponds to an alternator of the conventional engine-driven vehicle, however, the EV has no engine serving as a source of mechanical energy and the engine of the HV is not always operated because of the need for improvement of fuel consumption or because of the action of an idling stop system when the vehicle is stopped. Then, the auxiliary battery of the EV and HV is supplied with electric power from the main power supply (and also from a drive motor by power generation from regenerative braking when the vehicle is braked).
The above-mentioned auxiliary loads all operate at a low voltage from ten to twenty volts and accordingly the auxiliary battery outputs a voltage of a corresponding level. On the other hand, the main power supply serving as a power source of the vehicle usually outputs a voltage of a few hundreds of volts. In general, therefore, a DC/DC converter is provided for stepping down the voltage supplied from the high-voltage main power supply into the voltage of the auxiliary, in order to charge the auxiliary battery.
Japanese Patent No. 3,141,779 discloses a motor drive system for a vehicle of the series hybrid type that has such a DC/DC converter as described above. Specifically, an auxiliary battery is connected via the DC/DC converter to a main battery, and the DC/DC converter steps down a power supply voltage which is output from the main battery into an appropriate voltage of the auxiliary to apply the resultant voltage between positive and negative terminals of the auxiliary battery.
Suppose that a system includes such a DC/DC converter connecting the main power supply and the auxiliary battery and the system cannot be started due to a lowered power-supply ability of the main power supply after a long nonuse period. In such a case, the auxiliary battery may charge the main power supply via the DC/DC converter. However, the DC/DC converter is provided to the above-described system for the purpose of stepping down the high-voltage electric power supplied from the main power supply or the high-voltage electric power generated by the drive motor in the regenerative braking, into an appropriate voltage of the auxiliary loads that is supplied to the auxiliary battery. In general, therefore, the DC/DC converter of this type has no step-up function. Then, a conventional method for charging the main power supply by means of the auxiliary battery uses a separate step-up converter.
The method, as discussed above, of using the external charger for converting the AC voltage of the external commercial power supply into the DC voltage to charge the main power supply, requires the external charger having the AC/DC converting function, and thus the whole cost cannot be reduced.
The above-discussed system of providing the DC/DC converter between the main power supply and the auxiliary battery also requires a separate step-up converter, and thus reduction of the whole cost is impossible, as well.
Further, if a DC/DC converter having both of the step-up and step-down functions is used for allowing the auxiliary battery to charge the main power supply by controlling the step-up function when the power stored in the main power supply by charging decreases, the multifunctional DC/DC converter having the step-up and step-down functions adds the cost.